Juha Köykkä - Oulu

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Res Terrae, Ser. A 32, J. <strong>Köykkä</strong>, Sedimentology of the Mesoproterozoic Telemark basin-fills, South Norway: implications for sedimentation processes, depositional environments and tectonic evolution the Phanerozoic sedimentation models differ, at least partly, from the Precambrian models and systems, probably due to intensive weathering, rapid basin subsidence, and the lack of binding vegetation. Therefore, the established lithofacies scheme used in this thesis was modified for local sedimentation model purposes. The lithostratigraphic mapping and lithofacies analysis can be seen as an essential component of Precambrian sequence stratigraphy. Sequence stratigraphy allows an improved understanding of stratigraphic cyclicity and allogenic controls in sedimentary basin-fills, and allows the recognition of single or multiple cycles within sedimentary environments or within an entire basin (see Papers III and IV). The papers suggest that these cycles and their bounding surfaces are closely associated with changes in accommodation space and that they correspond with fluctuating water levels and changes in the base level. The limitations of the outcrop data in Precambrian sequence stratigraphy is mainly related to preservation potential and amount of data available (see Catuneanu and Eriksson, 2007; Catuneanu et al., 2009). The clastic petrofacies analysis can be an effective tool as supplementary data for geochemistry, and it was used in Paper II. The main limitations of the method on Pre- cambrian sedimentary rocks are biasing of the detrital framework and composition due to pseudomatrix, diagenesis, and multiple provenance sources, mixed transport history, paleoclimate, and metamorphosis. However, these problems can be partly avoided by using a combination of whole-rock geochemistry for tectonic reconstruction and detrital zircons for provenance studies on the same sedimentary deposits, as in Paper II. The problems with the Gazzi-Dickson modal analysis method are mainly related to cases in which there is a true mineralogical difference in grain size (Ingersoll et al., 1983). In fact, no point counting methods can adequately address this problem. How- ever, the method does allow the maximization of source rock data, and counting of poorly sorted and coarse-grained sandstones is fast. As noted by Ingersoll et al. (1983), the modal composition does not change due to the simple breakage of grains; that is, the weathering of grains still produces the same minerals, no matter the grain size. The modal analysis in Paper II shows that modal analysis results correspond with the geochemical results and the tectonic evolution of the basin. Geochemistry still is an important tool in ancient basin analysis. In particular, the combination of facies analysis, clastic petrofacies analysis, and geochemical data of 62
Res Terrae, Ser. A 32, J. <strong>Köykkä</strong>, Sedimentology of the Mesoproterozoic Telemark basin-fills, South Norway: implications for sedimentation processes, depositional environments and tectonic evolution siliciclastic sedimentary rocks provides important information about the lithostratigraphic correlation, weathering, provenance, paleoclimate, and paleotectonic settings of ancient sedimentary basins. However, the chemical composition of the provenance is probably the major control on the geochemistry of sedimentary rocks. This information can be distorted and biased by hydraulic sorting, post-depositional diagenetic reactions, metamorphosis, and weathering. XRF analysis, which was used in Papers I and II, is important for determining the paleogeographical model and tectonic setting and for comparing results from previous local studies. The main problems associated with the analyses were probably related to post-depositional diagenesis, metamor- phisms, and weathering. These issues were mainly reflected as a potassium addition by metasomatism. Therefore, it is important to identify the different mechanisms before evaluating geochemical data and building tectonic models for ancient sediments. The LA-ICP-MS techniques have permitted rapid in situ analysis of Lu-Hf isotopes and trace elements from zircon to further assist in determining provenance. Precam- brian shield areas are often extensively soil-covered and vegetated. Furthermore, the rocks themselves can be highly deformed, which hinders detailed sedimentological ob- servations, presenting a severe obstacle for Precambrian sedimentology and basin analysis. Therefore, isotope geochemical analyses are a “must” for fully understanding ancient sedimentary basin evolution. The main problems in Paper IV were likely related to the lack of comparable Lu-Hf data from the possible provenance sources, al- though a successful plate reconstruction and provenance history was achieved in Paper IV. Paper IV shows that it is important to fully understand the lithofacies assemblages and depositional environment where the detrital zircon sample is taken. Only this knowledge allows the building of coherent basin models and an evaluation of the provenance histories of the basin. Many of the modern basin types are rarely found in the ancient record because of uplift, erosion, and/or deformation and destruction. In addition, sedimentary basin subsidence history is difficult to estimate due to the limited use of the backstripping method and limited seismic data for ancient basins. The use of backstripping and seismic data would allow the removal of the sediment load from basin to reveal the tectonic driving mechanism of the basin’s subsidence history. The recognition of ancient sediment basins is usually based on the study and usage of lithofacies assemblages, se- 63
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Juha Köykkä - Oulu

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